Semiconductor devices such as an LED or a power module have a structure in which a semiconductor element is joined onto a circuit layer formed of a conductive material.
In a power semiconductor element for high power control which is used to control wind force power generation, electric vehicles, hybrid vehicles, and the like, the amount of heat generated is great, and thus, as a substrate mounting this power semiconductor element for high powder control, for example, a ceramic circuit substrate (a substrate for a powder module) including an insulating layer formed of a ceramic substrate of aluminum nitride (AlN), alumina (Al2O3), or the like and a circuit layer formed by disposing a metal having an excellent conductive property on first surface of this insulating layer has been broadly used in the related art.
For example, a power module (a semiconductor device) described in Patent Document 1 has a structure in which a substrate for a power module (a ceramic circuit substrate) having a circuit layer formed of a metal such as Al or Cu formed on a first surface of a ceramic substrate and a semiconductor element that is joined onto this circuit layer are provided. In addition, the power module has a constitution in which a heat dissipation plate is joined to the other side of the substrate for a power module and heat generated in the semiconductor element is transmitted toward the substrate for a power module and diffused to the outside through the heat dissipation plate.
When an electronic component such as a semiconductor element is joined onto a circuit layer, for example, a method in which a solder material is used as described in Patent Document 1 is broadly used. In recent years, from the viewpoint of environmental protection, for example, a lead-free solder such as a Sn—Ag-based solder material, a Sn—In-based solder material, or a Sn—Ag—Cu-based solder material has become mainstream.
Here, in a circuit layer formed of aluminum or an aluminum alloy, a natural oxide film of aluminum is formed on the surface, and thus it has been difficult to favorably join the circuit layer to a semiconductor element using a solder material.
In addition, in a circuit layer formed of copper or a copper alloy, there has been a concern that a molten solder material and copper may react with each other, a component of the solder material may intrude into the inside of the circuit layer, and the characteristics of the circuit layer may deteriorate.
Therefore, in the related art, a semiconductor element has been joined to the surface of a circuit layer using a solder material after a Ni-plated film is formed on the surface of a circuit layer as described in Patent Document 1.
Meanwhile, as a joining method in which no solder material is used, for example, Patent Document 2 proposes a technique for joining a semiconductor element using a Ag nano-paste.
In addition, for example, Patent Document 3 and 4 propose techniques for joining a semiconductor element using an oxide paste including metallic oxide particles and a reducing agent formed of an organic substance.
In addition, for example, Patent Document 5 to 7 propose a technique in which a Ag underlayer is formed on a circuit layer formed of aluminum or copper using a glass-containing Ag paste and then the circuit layer and a semiconductor element are joined together through a solder or the Ag paste. In this technique, the glass-containing Ag paste is applied and sintered on the surface of the circuit layer formed of aluminum or copper, whereby an oxide coating formed on the surface of the circuit layer is reacted with glass and removed so as to form a Ag underlayer, and the semiconductor element is joined onto the circuit layer on which the Ag underlayer is formed through the Ag joint layer formed of a sintered body of the solder or the Ag paste.
Here, the Ag underlayer includes a glass layer formed by the reaction between the glass and the oxide coating on the circuit layer and a Ag layer formed on the glass layer. In this glass layer, conductive particles are dispersed, and due to these conductive particles, the conductive property of the glass layer is ensured.